The expression of genetically controlled Ig Markers (allotypes) by Ig-bearing cells has become a fundamental problem in gene expression. The reigning theory is that Ig-bearing lymphocytes of allotypically heterozygous rabbits have only the capacity to express one Ig allotypic marker (allelic exclusion). The studies now underway are designed to test this hypothesis thoroughly. Data so far obtained are not consistent with the allelic exclusion hypothesis. Characterization of the mitogen responses and markers of rabbit lymphoid cells with different functional properties should provide needed fundamental knowledge of basic murine response mechanism. In general, the understanding of cellular function in immune responses is now dependent almost entirely upon data obtained from murine systems. A thorough understanding of the role of different lymphoid cell types in another species should help clarify which interpretations of the mouse system are applicable to other species and which are unique to the mouse. Further studies on the activation of rabbit lymphocytes and the distribution of cell surface receptors for mitogens during activation may contribute to an understanding of the control of cell proliferation and differentiation.